September 11th or 9/11 stands out in our
minds for obvious reasons. However there was another 9/11, 11th September
1947, when a crankcase explosion on the Reina
del Pacifico killed 28 men and injured 23 and led to the development
of crankcase relief valves and oil mist detectors. Of course there
had been crankcase explosions before this, but none which had such devastating
consequences.

Between 1990 and 2001 143 crankcase
explosions were reported to Lloyds Register which have about 20% of the
worlds shipping in its class, so if we use that as a factor, we can
estimate the total reported incidents were 715 in 11 years or about 65 a
year. Don't forget that these are reportable incidents, i.e. those where
the damage sustained has warranted a major repair or has resulted in
injury. Minor explosions may have gone unreported, and it is possible
that the actual number of incidents is more than double those reported.
- maybe 3 a week!!

Of those incidents reported to Lloyds, 21
explosions happened in two stroke marine diesel engines and 122 in four stroke
marine diesel
engines. But this doesn't mean that four stroke engines are more
likely to have an explosion; there are 7 times as many four stroke
engines at risk than two stroke engines.

SEQUENCE OF EVENTS LEADING UP TO A CRANKCASE EXPLOSION

For an explosion to occur there must be a source of air (oxygen),
fuel and ignition. Oxygen is present in the crankcase, but the
lubricating oil splashing around in the crankcase is in too large
droplets to start burning at the speed needed to cause an explosion, and
the oil/air concentration is too weak.

If,
however a mechanical fault develops with the consequent rubbing of
moving parts, then a hot spot will occur. This could happen in the
crankcase, chaincase, or camcase. When the temperature of the hot spot
reaches 200°C the lubricating oil splashing on to this hot spot
vapourises. The vapour then circulates to a cooler part of the crankcase
where it condenses into a white oil mist. The oil droplets in this oil
mist are very small - 5 to 10 microns in diameter. When the
concentration of oil mist reaches 50mg/l (about 13% oil mist - air
ratio), it is at its lower explosive limit. If this oil mist is now
ignited by the hot spot - and tests have shown that it is necessary for
a temperature of about 850°C to ignite oil mist in a crankcase under
operating conditions - then an explosion will occur.

Although the most common cause of of a localised hotspot is due to
friction, it is not the only cause of a crankcase explosion. A cracked
piston crown, blowby or an external fire have caused crankcase
explosions in the past.

PRIMARY AND SECONDARY CRANKCASE EXPLOSIONS

Severity of
explosions vary between a puff which may lift a relief valve to a
violent explosions which causes major damage and may injure personnel
and cause a fire. Evidence indicates that the longer the combustion
path, the more violent the explosion. This has become an area of concern
with the large two strokes of today which may have a crankcase volume of
500m3 +.

When an explosion occurs a
flame front travels down the crankcase with a pressure wave in
front of it. The turbulence caused by moving engine components causing
churning and mixing of vapours increase the speed of the flame front and
its area, which contribute to the increase in pressure. Turbulence
caused by venting of the pressure through relief valves can also
influence the explosion.

Following the venting of the explosion through the relief valves, there
is a drop in crankcase pressure to below atmospheric pressure. This can
cause air to enter the crankcase resulting in another flammable mixture
to be developed resulting in a secondary explosion to occur. The
secondary explosion is more violent and can result in crankcase doors
being blown off the engine, and fires starting in the engine room. If
the relief valves do not reseal after lifting, or if they do not lift at
all in the primary explosion ( due to lack of maintenance etc), then
door(s) may be blown off in the primary explosion, giving a ready path
for the ingress of air, which will make a secondary explosion more
likely. Air can also be sucked in via the crankcase vent, although rules
state that this must be as small as practicable and new installations
must have a non return valve fitted.

If a primary explosion occurs,
the pressure wave may send a large amount of oil mist out into the
engine room. Although the flame arrestors on the relief valves should
prevent ignition of this oil mist by the flame front, the mist will be
sucked up towards the turbocharger where it may be ignited by an
unlagged hot exhaust manifold. This ignition of oil mist can cause
severe damage to plant and personnel.

CAUSES OF CRANKCASE EXPLOSIONS

The table below
gives details of a number of accidents which have occurred since 1995 to
large slow speed 2 stroke engines where the cause is known. In a number
of cases death or serious injury to members of the crew occured.

Year

Cause of Explosion

Cause of Failure

1995

Bearing in PTO gearbox

1996

Inlet pipe for piston
cooling oil falling off

Incorrect tightening

1997

Incorrect spring
mounted in piston rod stuffing box

Unauthorised spare
part

1997

Piston rod
interference with cylinder frame

1999

Weight on
chain tightener falling off

Incorrect
tightening

1999

Fire
outside the engine

2000

Main
bearing

2000

Camshaft
bearing

2000

Incorrect
shaft in camshaft drive

Unauthorised spare part

2001

Crankshaft
failure

2001

Piston
crown failure

2001

Main
bearing

2001

Crankpin
bearing

2001

Inlet pipe for piston
cooling oil falling off

Incorrect tightening

This
article and the following articles can be found in the
members
section under